1. Field of the Invention
This invention relates to a porous compound oxide to be suitably used as catalyst or adsorbent or as a carrier thereof and, more specifically, it relates to a spinel type compound oxide expressed by general formula MO--Al.sub.2 O.sub.3 (M representing a metal atom with a valence of 2).
2. Related Background Art
The spinel structure refers to a typical crystal structure observable in double oxides having a composition of AB.sub.2 O.sub.4 (both A and B representing a metal element). The denomination of spinel derives from that of a mineral compound (a double oxide of magnesium and aluminum expressed by chemical formula MgAl.sub.2 O.sub.4) showing the above structure and determined to be as such for the first time. A compound having a spinel structure forms a crystal showing an octahedral profile that belongs to the cubic lattice (having eight chemical units of AB.sub.2 O.sub.4 in a unit lattice) containing oxygen atoms substantially to the cubic closest packing (The Large Chemical Dictionary: Kyoritsu Shuppan, vol.5, p. 171). Compounds having the spinel structure are stable over a wide temperature range and remain stable in a relatively hot environment so that such compounds are used for sealing materials to be used at high temperature (see, for example, Japanese Patent Application Laid-Open No. 9-153369). Porous spinel type oxides, on the other hand, are popularly used for catalysts and adsorbents and also as carriers thereof. Japanese Patent Application Laid-Open No. 9-225267 discloses magnesium oxide-copper oxide-aluminum oxide spinel that can be used for catalytically reducing the nitrogen oxide contained in exhaust gas discharged from internal combustion engines and thermoelectric power plants. Japanese Patent Application Laid-Open No. 9-192490 discloses an alumina type compound oxide containing at least an element selected from cobalt, nickel and zinc and having a spinel structure in order to provide an exhaust gas purifying catalyst showing a heat resistance and an activation promoting effect much more excellent than known alumina type compound oxides. Japanese Patent Application Laid-Open No. 9-276699 discloses a hydrogenation catalyst precursor containing as principal ingredient a compound oxide having a copper-aluminum spinel structure, a hydrogenation catalyst using the same and a method of manufacturing alcohol using such a hydrogenation catalyst in order to provide a hydrogenation catalyst precursor free from the problem of environmental contamination of copper-chromium catalysts and more excellent than known copper-iron-aluminum catalysts in terms of activity, durability and selectivity as well as a hydrogenation catalyst and a method of manufacturing alcohol using such a catalyst.
When using a porous spinel type oxide as catalyst or adsorbent or a carrier thereof, the prepared catalyst or adsorbent or the carrier thereof importantly has to show a large surface area in order to raise its activity and adsorbing ability. Additionally, when it is used as catalyst, it is required to have a uniform surface in terms of characteristic properties in order to enhance its selectivity and suppress degradation of its activity. Therefore, it is required to show little change during its use in terms of surface area and characteristic properties. Now, spinel type oxides are structurally relatively stable over a wide temperature range and hence show little change in terms of surface area and characteristic properties in use so that they can advantageously be used in a high temperature environment. However, in order for spinel type crystal to grow, generally it will have to be calcined at high temperature. If the material to be calcined is porous, it may not be able to maintain a large surface area and a uniform micro-porous structure under such a high temperature condition and consequently the surface area will inevitably become reduced in use. Currently, this problem can be avoided only by using it by a relatively large volume. Thus, there is a demand for a porous spinel type oxide showing a large surface area that is made of a material whose surface area may not be reduced remarkably if it is calcined at high temperature to make it show a spinel structure.